2000 Toyota Corolla trouble codes.

I have an engine scanner. The trouble coded I downloaded from my corolla is PO171, PO303, PO304, P0303. My car check engine light has been steadily on for 3 years now Car seems to run fine but gas mileage performance seems to have dropped.
I changed the o2 sensor after the check engine light came on back then. The light went away but came back on a few days after and has remain ON.

REcently I checked the o2 sensor on the voltage. The resistance and voltage were below spec. I notice a suction sound near the radiator. Appreciate any help.

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2000 Toyota Corolla trouble codes.

Technical Description

System Too Lean (Bank 1)What does that mean?

Basically this means that an oxygen sensor in bank 1 detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1.
Note: This DTC is very similar to P0174, and in fact your vehicle may show both codes at the same time.Symptoms

You will more than likely not notice any drivability problems, although there may be symptoms such as a lack of power, detonation (spark knock), and/or a hesitation/surge on acceleration.Causes

The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.

There could be a vacuum leak downstream of the MAF sensor.

Possible Solutions

Possible solutions include:

In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling

Inspect all vacuum and PCV hoses, replace if necessary

Check for a dirty fuel filter and proper fuel pressure

P0303 Code - Cylinder #3 Misfire

Technical Description

Cylinder #3 Misfire DetectedWhat does that mean?

A P0303 code means that the the car's computer has detected that one of the engine's cylinders is not firing properly. In this case it's cylinder #3.Symptoms

Symptoms may include:

the engine may be harder to start

the engine may stumble / stumble, and/or hesitate

other symptoms may also be present

Causes

A code P0303 may mean that one or more of the following has happened:

Faulty spark plug or wire

Faulty coil (pack)

Faulty oxygen sensor(s)

Faulty fuel injector

Burned exhaust valve

Faulty catalytic converter(s)

Running out of fuel

Poor compression

Defective computer

Possible Solutions

If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

P0304 Code - Cylinder #4 Misfire

Technical Description

Cylinder #4 Misfire DetectedWhat does that mean?

A P0304 code means that the the car's computer has detected that one of the engine's cylinders is not firing properly. In this case it's cylinder #4.Symptoms

Symptoms may include:

the engine may be harder to start

the engine may stumble / stumble, and/or hesitate

other symptoms may also be present

Causes

A code P0304 may mean that one or more of the following has happened:

Faulty spark plug or wire

Faulty coil (pack)

Faulty oxygen sensor(s)

Faulty fuel injector

Burned exhaust valve

Faulty catalytic converter(s)

Running out of fuel

Poor compression

Defective computer

Possible Solutions

If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

2000 Toyota Corolla trouble codes.

In the absence of a vac leak, mass airflow meters go bad on these once they get past 100,000 miles. I do atleast one a month at my dealership. Not a cheap part, but easy to do yourself with just a Philips screwdriver.